Excitation of nucleon resonances in

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Excitation of nucleon resonances in
charge-exchange reactions
J. Benlliure
Universidad de Santiago de Compostela Spain
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Motivation
Charge-exchange reactions are unique probes to
investigate spin-isopin properties of nuclei. -
Contrary to b-decay, these reactions may excite
spin-isospin modes over a broad range in
energy. - Spin-isospin excitations manifest in
two different energy domains

• at low energies particle-hole excitations
  (Gamow-Teller, spin-dipole, spin- quadrupole or
  quasi-elastic).
• - Gamow-Teller BGT transition strengths
• - spin-dipole radial distributions of
  protons and neutrons

A. Krasznahorkay et al. NPA 731 (2004) 224

• at high energies excitation of nucleon
  resonances (e.g. D resonance)
• - In-medium properties of baryon resonances
  (downward shift of the D mass
• when excited in nuclei) .

C. Bachelier et al. PLB 172 (1986) 23
Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Motivation
Subnucleonic degrees of freedom play a role in
ground-state and structural properties of nuclei.

• Gamow-Teller strength quenching.
• role of D-h excitations

• Three-body nuclear forces.
• role of D(1232) and Roper(1440)

The relative probability for inelastic (n,p) and
(p,n ) reactions can be used to prove the
abundance of protons and neutrons at the nuclear
periphery.

• Because of the strong absorption isobar charge-
• exchange reactions are peripheral processes.

Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Motivation
To investigate spin-isospin excitations at
nuclear and subnucleonic level in asymmetric
nuclear matter inducing isobar charge-exchange
reactions, (p,n) and (n,p), on radioactive
relativistic projectiles
The momentum recoil induced by the pion emission
proves the excitation of the resonance.
Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Experimental requirements

• Quasi-elastic and inelastic isobar
  charge-exchange reactions, (p,n) and (n,p), in
  asymmetric nuclear matter
• relativistic heavy-ion collisions induced by
  exotic projectiles (isospin
• asymmetry and radial distributions of
  protons and neutrons)
• isobar charge-exchange (clean reaction channel)

• Observables
• cross sections for both charge-exchange
  reactions and channels
• missing-energy spectra of the projectile residues

• Requirements for the setup
• isotopic identification of relativistic
  projectile residues
• separation of elastic and resonant
  charge-exchange channels
• (magnetic analysis of the projectile
  residues)

Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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The experiment
Inverse kinematics
DE 25 MeV (124Sn _at_ 1000 MeV/u)
DZ/Z 7 10-3 DA/A 2.4 10-3
Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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The experiment

• Reactions investigated
• 124SnCH2,C, Cu, Pb ?124Sb,124In _at_ 1000 A MeV
• 124SnBe ? 120SnCH2,C ? 120Sb,120In _at_ 1000 A
  MeV
• 112SnCH2,C,Cu,Pb ?112Sb,112In _at_ 400, 700, 1000
  A MeV
• 112SnBe ? 110SnCH2,C ? 110Sb,110In _at_ 1000 A
  MeV

Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Results
Isotopic identification of isobaric
charge-exchange residues
Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Results
Missing-energy spectra in isobar charge-changing
reactions induced by 112Sn
(p,n)
(n,p)
Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Results
Missing-energy spectra in isobar charge-changing
reactions induced by 112Sn
C. Bachelier et al. PLB 172 (1986) 23

• The observed spectra are in good qualitative
  agreement with previous measurements at Saturne
• (France)

Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Results
Unfolding the missing-energy spectrum with the
experimental response function
 
Y measured spectrum H experimental response
function X observable
The primary beam centred through the
FRS constitutes our response function H
J. Vargas, J.B. and M. Caamaño NIMA 707 (2013)
16
Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Results
Unfolding the missing-energy with the
experimental response function
112Sn(63Cu,X)112Sb

• The unfolding procedure improves the resolution
  of the final spectra

Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Results
Excitation of nucleon resonances
112Sn(63Cu,X)112Sb

• Unfolded spectra show additional structures in
  the inelastic charge-exchange peak that can be
• associated to the excitation of different
  nucleon resonances as the D and Roper resonances.

Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Recent measurements with the FRS
Resolving nucleon resonances
Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Results
Energy and target dependence of the resonance
excitation
In nucleus-nucleus collisions D-resonance
excitation may occurs below the pion production
threshold - Fermi momentum of the nucleons
Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Results
Energy and target dependence of the resonance
excitation

• In nucleus-nucleus collisions D-resonance
  excitation
• occurs below the pion production threshold
• Fermi momentum of the nucleons
• Lower energy of the D-resonance

Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Results
Charge-exchange cross sections

• Both cross sections are sensitive to the neutron
  excess at projectile periphery s(n,p) gt s(p,n)
• and the projectile size s(124Sb) gt s(112Sb)

• Elastic (n,p) reactions in the projectile are
  inhibited in heavy targets by charge conservation

• Simple targets such as protons or carbons seem
  better suited to link these cross sections to the
  relative abundance of protons and neutrons at the
  projectile periphery but model calculations are
  required

Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Model calculations
Differential cross sections
Isaac Vidaña, U. Coimbra
Free NN cross sections
Scattering amplitudes
target excitations ..
Vertices
Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Model calculations
Isaac Vidaña, U. Coimbra
Preliminary calculations

• Preliminary model calculations provide a good
  qualitative description of the measured
  distributions

• The calculations clearly show the contribution of
  different resonances but also their excitation in
  the projectile but also in the target nucleus

• Measurements with proton and carbon targets will
  be used to extract the properties of the observed
  resonances

Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Future plans at
Experimental programme at the Super-FRS
Projectile excitations

• Exclusive measurements
• Pion detection
• Projectile and target excitations
• Identification of resonance isobars

Target excitations

• Measurements with secondary beams
• Higher secondary beam intensities
• Contaminants suppresion
• Thinner reaction targets

Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Conclusions

• A high-resolving power magnetic spectrometer has
  been proven to be an excellent tool to identify
• nucleonic excitations in heavy-ion
  charge-exchange reactions
• - several baryon resonances were identified
  in the projectile missing energy spectra
• - cross sections for the elastic and
  inelastic (p,n) and (n,p) channels where also
  measured
• - in-medium properties of these baryons and
  relative abundance of protons and neutrons at the
  
• nuclear periphery are investigated using
  reliable model calculations

• The Super-FRS at FAIR will offer unique
  opportunities for these investigations
• - extremely asymmetric nuclear matter
• - improved separation capabilities
  (pre-separator) and resolution
• - exclusive measurements detecting pions in
  coincidence

Collaborators U. Santiago de Compostela J.
Vargas, Y. Ayyad, S. Beceiro, D. Cortina, P.
Díaz, M. Mostazo, C. Paradela GSI T. Aumann, J.
Atkinson, K. Boretzky, A. Estrade, H. Geissel, A.
Kelic, Y. Litvinov, S. Pietri, A. Prochazka, M.
Takechi, J. Winfield CEA/DAM A. Chatillon, J.
Taieb U. Coimbra I. Vidaña U.
Giessen H. Lenske
Tokyo, Japan, June 2014
José Benlliure, ARIS 2014
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Recent measurements with the FRS
Missing-energy spectra with secondary beams
112Sn
110Sn
110Sn(C,X)110Sbp-
110Sb
Main difficulties - Low statistics - Thick S2
target (1 g/cm2 C) - Important background at S2
José Benlliure
SuperFRS collaboration meeting, October 2013
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Recent measurements with the FRS
Unfolding and regularization of the
missing-energy spectrum
The most reliable unfolding techniques are based
on iterative procedures (Richardson-Lucy)
Regularization methods are needed to determine
the optimum number of iterations
J. Vargas, J.B. and M. Caamaño NIMA 707 (2013)
16
José Benlliure
SuperFRS collaboration meeting, October 2013
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Proposal
To investigate the isospin dependence of
spin-isospin excitations at low and high momentum
transfer for both isobar charge-exchange channels
(p,n) and (n,p) using relativistic exotic
projectiles.

• In-medium properties of baryon resonance in
  isospin asymmetric nuclear matter. (mean energy
  and width of the resonance)

• Gamow-Teller transition strengths

• Radial distributions of neutrons and protons.

• Nuclear matrix elements for inelastic neutrino
  interactions.

José Benlliure
SuperFRS collaboration meeting, February 2014